This invention relates to an exhaust gas recirculation method for internal combustion engines, which is capable of achieving stable operation of the engine under low atmospheric pressure such as at high altitude.
Exhaust gas recirculation, i.e so-called EGR, is widely employed in internal combustion engines wherein part of exhaust gases from the engine is returned to the intake passage of the engine so as to reduce nitrogen oxides (NOx), one of noxious gases emitted from the engine. It has also been widely employed to detect the opening of an exhaust recirculating valve arranged in an exhaust gas recirculating passage, and control the exhaust gas recirculating valve so that the detected valve opening value becomes equal to a desired valve opening value to thereby attain optimal amounts of exhaust gas recirculation approriate to operating conditions of the engine.
Conventionally known actuator means for actuating the exhaust gas recirculating valve include a differential pressure-operated type which typically comprises a diaphragm connected to the exhaust gas recirculating valve, an atmospheric pressure passage for guiding atmospheric air or atmospheric pressure to one side surface of the diaphragm, a negative pressure passage for guiding vacuum or negative pressure developed in the intake passage of the engine at a location downstream of a throttle valve therein, to the other side surface of the diaphragm, and a second atmospheric pressure passage for guiding atmospheric pressure to the other side surface of the diaphragm, a solenoid valve arranged across the negative pressure passage or the second atmospheric pressure passage, and a control circuit for supplying a control signal to the solenoid valve for controlling same, wherein the diaphragm is displaceable in response to a pressure difference between atmospheric pressure applied to the one side surface thereof and a synthetic pressure formed by the negative pressure and atmospheric pressure, applied to the other side surface thereof, to thereby control the opening of the exhaust gas recirculating valve.
With the above construction of the actuator means for the exhaust gas recirculating valve, however, when the engine is operating at a high altitude, the pressure difference between pressures applied to the both side surfaces of the diaphragm is small due to low atmospheric pressure, often causing the phenomenon that even when the solenoid valve is supplied with a control signal commanding opening of the exhaust gas recirculating valve from the control circuit, the diaphragm is not displaced through a required stroke at all, so that the actual valve opening of the exhaust gas recirculating valve does not approach the desired or command value. As a result, at such high altitude, it is not possible to reduce the amount of NOx in the exhaust gases. Besides, since even on such occasion fuel is supplied to the engine in an amount commensurate with the desired command value of exhaust gas recirculation, a lean air-fuel mixture will be supplied to the engine if the exhaust gas recirculating valve is not opened to the desired command value, often causing unstable operation of the engine. Moreover, the solenoid valve is continually supplied with energizing current from the control circuit even when the exhaust gas recirculating valve is not opened to the desired command value, badly affecting the effective life of the solenoid valve.